Using a UI Framework like Semantic UI when building webpages seem pretty much mandatory in this day and age. There is a hurdle in learning how to use an API and its features, but the payoff is worth it. UI Frameworks provide the developers with the ability to create more robust and visually appealing webpages. Using raw HTML and CSS to create a webpage of similar quality as one built with Semantic UI would require a lot more time and effort. It would be like writing programs in extremely low-level languages. Unless you need some highly specific and unique feature, it’s much more cost-efficient to make use of the work that other developers have done in creating a library/API with useful features. I’m sure that it is possible to write in HTML an element that behaves like a Semantic UI grid/column, but why bother trying to reinvent the wheel. Chances are, simple features and elements that you want to incorporate into your webpage have already been implented by someone, and you can just look at the documentation and use them.
I do not have much experience with HTML or CSS, but so far, it definitely seems to me that there are severe limitations to the kinds of webpages you can make using just HTML and CSS, especially in regards to how they look visually. Semantic UI was somewhat easy to pick up and start using, and the benefits of it are very clear. The whole point of a framework is that it is meant to be intuitive. You don’t necessarily have to know how or why something works, just that it does. All you really need is to look at the documentation and some examples to get started building webpages that are more visually appealing. I like to think of frameworks and API’s to be like the raw materials that craftsmen use to build things. Rather than using primitive materials like wood and stone, a craftsman can build much better things with smelted and refined metals created with the skills and knowledge of someone who specializes in metals.